gstreamer1.0-plugins-bad: clean up X11 dependencies

libxcb and xkbcommon are only needed in builds with X11 enabled, so add
a PACKAGECONFIG and respect the x11 DISTRO_FEATURE.

Strictly speaking these are only required if Vulkan and X11 are enabled,
but it's possible that more plugins will respect the flag.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>